Coccidiosis is a parasitic disease of the intestinal tract of chickens. This disease has become widespread among commercial poultry institutions and is responsible for the death of millions of chickens each year. Chickens that survive the disease are often emaciated and sickly and are unsuitable for food consumption or egg laying for a prolonged period of time.
Mycoplasma gallisepticum is an organism that causes a chronic respiratory disease in poultry. This disease, like coccidiosis, is responsible for severe economic losses within the poultry industry.
Coccidial infections of the intestinal tract and mycoplasmal infections of the respiratory tract of the chicken occur primarily in epithelial cells of the mucosae. The organisms do not invade deeply enough to cause systemic infections. Consequently, the main line of defense should be at the mucosal surface in the form of secretory antibody or cell-mediated immunity. Attenuated strains of coccidia have been administered orally to chickens, but only partial immunization to pathogenic coccidia has been achieved.
Although the preferred method for the control of mycoplasma in poultry flocks is eradication of the organism in the poultry flocks, vaccination is another possible means to control the disease. An attenuated strain of mycoplasma has been used to vaccinate chickens by respiratory exposure. However, the attenuated strain does cause mild, transitory disease. Furthermore, the attenuated vaccine persists in the environment. The U.S. Department of Agriculture has been reluctant to approve such an attenuated vaccine because of its prolonged existence in the environment and the potential to revert to a virulent strain.
The U.S. Department of Agriculture has approved several vaccines comprising inactivated or killed mycoplasma, but, because these vaccines are administered parenterally, mucosal immunity is not adequately stimulated. Even though losses are reduced by vaccination, infection and the persistence of pathogenic strains occur.
What is needed is a vaccine comprising a non-viable organism or part of an organism that can stimulate adequate levels of mucosal immunity. Adequate stimulation should best be achieved by inoculation of the mucosal surfaces. However, numerous attempts to do this by exposing respiratory mucosae to killed mycoplasma have failed. Traditional adjuvants cannot be used with these vaccines because they would cause excessive irritation of the extremely sensitive respiratory and conjunctival mucosae.
Therefore, an improved method of vaccinating poultry by successfully exposing mucosae to a vaccine preparation is needed.